Preparation of silver-metal azide ignition composition

ABSTRACT

A PROCESS FOR MANUFACTURING AN IGNITION COMPOSITION HAVING UNIFORM, RIPID, LOW-ENERGY IGNITION RESPONSE TO LOW LEVELS OF VOLTAGE COMPRISES THE IN SITU REDUCTION BY FINELY-DIVIDED MAGNESIUM OF SILVER NITRATE DISSOLVED IN AN AQUEOUS SUSPENSION OF FINELY-DIVIDED SILVER AZIDE OR LEAD AZIDE. THE PRODUCT COMPOSITION, AN INTIMATE MIXTURE OF SILVER AND AZIDE PARTICLES, IS RECOVERED BY FILTRATION OR DECANTATION, WASHING AND DRYING.

Int. Cl. C06c 1/02 U.S. Cl. 149-35 9 Claims ABSTRACT OF THE DISCLOSURE A process for manufacturing an ignition composition having uniform, rapid, low-energy ignition response to low levels of voltage comprises the in situ reduction by finely-divided magnesium of silver nitrate dissolved in an aqueous suspension of finely-divided silver azide or lead azide. The product composition, an intimate mixture of silver and azide particles, is recovered by filtration or decantation, Washing and drying.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of my prior copending application Ser. No. 219,841, filed on Jan. 21, 1972, now abandoned, which in turn is a continuation-inpart of my application Ser. No. 51,333, filed on June 30, 1970, now abandoned.

BACKGROUND OF THE INVENTION Certain applications for electro-explosive devices require that the ignition system of such devices respond in very short periods of time to very low levels of electric energy stimuli. Typical applications for such devices are found in some types of explosive fasteners, shaped charge initiators, and military fuzes, and these applications usually require also that the device he miniature in size, i.e., considerably smaller than the typical commercial electric blasting cap. Small, fast-firing, low-energy electro-explosive devices are known in the art and are exemplified in U.S. Pats. 3,332,311 to the present applicant and 2,918,- 871 to D. D. Taylor. U.S. 3,332,311 teaches an electrically actuated, explosively expanded, nonrupturing, rivet type fastener and U.S. 2,918,871 teaches a miniature electric detonator, both of which use some form of lead azide mechanically mixed with a conductive material such as carbon as an ignition mixture through which the passage of electric current causes rapid ignition at low energy levels. U.S. 3,476,623 to Menke et al. teaches the use of a secondary (high) explosive mechanically mixed with an azide (primary) explosive and a conductive material such as graphite, carbon black, or colloidal silver as embodying desirable process and product features for low energy ignition applications. German Pat. 1,173,373 describes a process for manufacturing a conductive azide type explosive wherein the azide crystals are precipitated in the presence of a colloidal suspension of conductive material such as graphite, silver, or other colloidal metal or semiconductor and said azide crystals form on, but do not completely surround, the conductive colloidal particles. The claimed response times for the ignition mixtures cited in the above referenced patents range from 2 to 100 microseconds following the application of from 2 microjoules (20 ergs) to 50 millijoules (500,000 ergs) of energy depending on the ignition mix employed and such parameters as the size and shape of the electrode-conductivemix-configuration and the compression of the conductive IIllX.

United States Patent O ice A limitation of the above cited prior art low-energy ignition materials is that, despite the achievement of the desired fast, low-energy response, the relation between ignition energy and the resistance of the current path in the devices employing these materials is such that at least 10 volts (and nominally 40 to volts) applied through the disclosed circuits, is required to deliver the current needed for even the low ignition energy levels described. Since many of the applications of low energy explosive devices require that the devices be miniature, it is also desirable in some instances that the power source for the device be miniature; e.g., a single, small 1.5 volt cell of the type commonly employed in compact, low-power circuits.

U.S. Pat. 3,155,553 to Taylor et al. describes a detonator having an ignition system responsive within the low ranges of time and applied energy already cited and at supply voltage levels as low as 1.5 volts, wherein the responsive ignition material comprises a mixture of lead azide and powdered gold. The patent teaches that the size of the gold particles in the mixture is quite critical and that the gold powder, which is received in the form of a dry precipitate, is subsequently wet-mixed in the presence of methanol or acetone with lead azide preferably of the crystalline form that has been obtained via precipitation in a solution of polyvinyl alcohol (i.e., so-called PVA lead azide). The mixed suspension of lead azide and gold is then milled to provide the required degree of fineness 16 microns) of the gold particles in the mixture and the milled mixture of lead azide and gold is subsequently separated from both the grinding and the liquid media. This process thus involves the steps of mixing, milling, and collecting the lead azide/gold combination in addition to the routine preparation of the PVA lead azide, the milling step being potentially hazardous or troublesome because of the known sensitivity of lead azide to dislocations within its crystalline particles even when the azide is wet.

In addition to the previously referenced prior art uses of silver in low-energy, conductive ignition materials, a method of coating lead azide particles with silver via reduction of silver nitrate with hydrazine hydrate is described in a report entitled Influence of Silver Coating on Ignition Behavior of Colloidal Lead Azide by B. Reitzner and publihed as.Technical Report FRL-TR-2, August 1960, by Picatinny Arsenal, Dover, N]. This study is concerned with the effects of the silver coating on autoignition of the lead azide at elevated temperatures and is not concerned with electric ignition per se. The aforementional U.S. 3,476,623, however, states that silver coatings of silver azide provided by the hydrazine reduction method yield nonhomogeneous material that is unsatisfactory as a low-energy (electrically) conductive ignition powder, which result has been independently substantiated by the present application.

It is thus a primary object of the present invention to provide an improved process for making an ignition mixture that responds rapidly to low-energy levels obtained from low-voltage sources and is homogeneous in composition.

It is another object of the invention to provide a process that is free of any particularly hazardous or troublesome steps in the production of the desired ignition material.

Still another object of the invention is to provide a process that yields a conductive ignition mixture of the desired rapid, low-energy, low-voltage response when employed in electro-explosive devices and which is yet no more sensitive than commonly used primary explosives (e.g., dextrinated lead azide) to impact and static electricity during explosives handling operations routine in the preparation of such devices.

3 SUMMARY OF THE INVENTION It has now been found that a conductive ignition mixture having the desired fast-acting, low-energy, low-voltage response can be prepared by the in situ precipitation of silver during the final stages of preparation of lead azide suspensions. According to the present invention there is provided a process for the preparation of an ignition composition comprising an essentially homogeneous particle mixture containing approximately 9045% particles of a lead azide and approximately -55% silver particles and having rapid response to ignition voltages below volts and usually below 10 volts, comprising reducing in situ a dissolved silver nitrate with finely-divided magnesium in the presence of a substantially aqueous suspension of a metallic azide selected from the class consisting of lead azide and silver azide to precipitate essentially all the silver from the nitrate as metal and thereafter recovering the product thus obtained.

DETAILS OF THE INVENTION The process of the present invention comprises the precipitation of particulate silver in the presence of a suspension of lead or silver azide wherein the silver thus precipitated comprises 10% to 55%, and preferably 25% to 35%, by weight of the total particulate mixture in the suspension.

Lead azide in a form suitable for preparation of the present compositions can be produced by precipitation with sodium azide of an aqueous solution of lead acetate, as in the manner long known in the art and described in US. Pat. 1,914,530 to W. H. Rikenbach which produces particles usually less than 10 microns down to colloidal size. During the final stages of the formation of the precipitated lead azide within the stirred aqueous system, i.e. after about 2-5 minutes of stirring the freshly precipitated lead azide suspension, silver nitrate (AgNO and finally powdered magnesium are added to the stirred suspension. Alternatively, the freshly precipitated lead azide can be washed one or more times with water by permitting the precipitate to settle, decanting the Water and reslurrying the lead azide. The silver nitrate and finely-powdered magnesium are then added. The addition of magnesium can be followed immediately by, immediately succeed, or be concurrent with the addition of the silver nitrate solution. The total amount of AgNO introduced into the suspension is determined by the batch size of the lead azide manufacturing process and the resultant percentage of silver desired in the lead azide/silver mixture. The amount of powdered magnesium in turn should be stoichiometric with the amount of AgNO that is added in order to assure essentially complete exchange of magnesium and silver and thus provide the desired percentage of precipitated silver with essentially no residue of metallic magnesium. Exact stoichiometry is not critical to the performance of the ignition mixture; a tolerance within 3% excess magnesium or deficiency of desired silver does not significantly affect performance. The mixed aqueous suspension of lead azide and silver is stirred, the mixed precipitate is allowed to settle, washed and finally recovered by filtration or decantation and dried.

The mixture thus prepared, when dried for use in detonators and other explosive devices, can be handled in the manner of other well-known primary explosives (lead azide alone, diazodinitrophenol, etc.). The sensitivity of the typical dry, uncompressed lead azide/ silver mixture of the preferred process to impact is on the order of the sensitivity of standard dextrinated lead azide and requires significantly higher average energy levels for static ignition than does dextrinated lead azide. The mixture of the present invention has an ignition sensitivity to energy on the order of 10 microjoules (0.01 millijoule, 100 ergs) to 250 microjoules (0.25 millijoule, 2500 ergs) when a voltage on the order of 3.0 to 15 volts is applied across the electrode and body of a miniature detonator using a 2.2 microfarad capacitor wherein the mixture is compressed to a value of about 12,800 p.s.i. The mixture of the present invention also will fire When a voltage on the order of 1.5 volts (DC) is applied across the electrode and body of an explosive rivet prepared in accordance with the teachings of US. 3,332,311 wherein the mixture is compressed to a value of about 90,000 p.s.i. (devices with other geometries may have other compression values, firing voltages and ignition sensitivities). The nominal response time of the ignition mixture under the described conditions is 10 microseconds.

Because of the fine size of the constituent particles of the ignition mixture it may be desirable in some applications to add a graining material such as nitrostarch, nitrocellulose, or certain naturally occurring gums that are known in the art in order to facilitate handling of the ignition mixture during such operations as the loading and pressing of the mixture into explosive devices. It has been found that the performance of the ignition mixture is not significantly affected it up to 5% by weight of graining agent is added to the total mixture. The graining agent, when used, is blended with the mixed precipitate of lead azide and silver and the resultant mixture is grained through a 38-mesh bolting cloth.

As-precipitated mixtures of the lead azide and silver perform satisfactorily as low-energy, low-voltage ignition mixtures with silver content ranging from 10 to 55%. Optimum performance is achieved in the preferred range of 2535% silver. The minimum voltage for ignition increases rapidly with increasing percentage of silver above 45%. Decreasing the percentage of silver below 25% produces gradually increasing firing voltage and firing times; however, even at 20% silver content, ignition is produced Within 100 microseconds by applied voltages as low as three volts. Thus the improved process of the present invention applies not only to the preferred mixtures wherein applied voltages as low as 1.5 volts produce fast response at relatively low-energy levels, but also to mixtures wherein the desired applied voltage levels may be above 1.5 volts.

Addition of a graining agent (nitrostarch) to mixtures having silver contents in the 30-45% range does not significantly affect ignition performance, but at silver proportions less than 30% the increases in required firing voltage and firing times are accelerated by the addition of the nitrostarch.

Fast-acting, low-energy, low-voltage conductive ignition mixtures can also be prepared by applying the process of this invention to other forms of lead azide such as dextrinated lead azide and certain special purpose (military) lead azides such as the so-called RD-1333 (military specification No. MIL-L-46225A, dated Mar. 29, 1963) lead azide. An ignition mixture of similar properties can also be prepared by using a silver azide suspension instead of lead azide. None of these alternative materials has an average voltage response level quite as low as the prepared lead azide/ silver mixture, yet all are ignitable at voltages under 15 volts. The optimum ratio of dextrinated lead azide to silver is /15, and this mixture requires, on the average, one volt more for ignition than the preferred lead azide/silver mixture. The dextrinated lead azide/silver mixture is an order of magnitude less static sensitive than the preferred mixture and of about equal impact sensitivity. The RD-1333 lead azide/silver mixture also has an optimum ratio of 85-15, requires about 1.5 volts more for ignition than the preferred mixture, has about equal static sensitivity and is only about half as impact sensitive. Mixtures containing from 10 to 40% silver give satisfactory low voltage ignition performance for either dextrinated or RD-1333 lead azide. The silver azide/silver mixture requires only about 0.5 volt more for ignition than does the preferred mixture and has about the same optimum azide/ silver ratio (60/40) and impact sensitivity, but is less economical to manufacture.

5 EXAMPLE 1 A suspension of lead azide was prepared by adding to 200 ml. of distilled water 10 ml. of a sodium azide solution containing 109.1 grams/ liter sodium azide plus about 0.12 gram of sodium carboxymethylcellulose and about 0.7 ml. of Empilan (a non-ionic surfactant, 10% by volume, monolaurate ester of polyethylene glycol, mean molecular weight of 400, imported from United Kingdom by Aceto Chemical Co., Flushing, N.Y.), and rapidly adding into this 10 ml. of a lead acetate solution (317.5 grams/liter) with vigorous stirring. The product was a suspension of a lead azide. After five minutes of stirring, 2.1 grams of silver nitrate in water solution were added. Immediately, 0.155 gram of magnesium powder, having a nominal particle size of five microns, was sifted into the mixture and stirring was continued for one hour. The precipitate was then allowed to settle and was washed by decantation using water followed by acetone. The precipitate was then filtered by gravity and dried under ambient conditions.

Photomicrographs of the precipitate showed that it consisted of a homogeneous mixture of crystals of lead azide approximately 2.5 to 5 microns in size and dendritic particles of silver metal approximately 0.1 to 10 microns in size. There was no evidence that either constituent tended to coat or agglomerate on the other.

The product was tested in the explosive rivet of U.S. 3,332,311. After compression under a pressure of approximately 90,000 p.s.i., the composition was ignited upon application of 1.5 volts direct current.

EXAMPLE 2 At room temperature, a suspension of lead azide was prepared by rapidly adding 401 ml. of a sodium azide solution containing 89.69 grams/liter sodium azide into 398 ml. of a lead nitrate solution containing 229.9 grams/ liter lead nitrate with vigorous stirring to assure fast mixing. The product was a suspension of a lead azide. After two minutes of stirring, 3.89 grams of magnesium powder in suflicient (S.D.A.-3A) denatured alcohol to permit suspension of the powder was poured slowly into the mixture. Immeidiately thereafter, 54.33 grams of silver nitrate in 200 ml. deionized water solution was added and stirring was continued for one hour. The precipitate was then allowed to settle and was washed twice with deionized water by reslurrying and decantation followed by two washings with denatured alcohol by reslurrying and decantation. The precipitate was then dried under ambient conditions.

The composition was tested by fabricating and firing ten electrical detonators. 'Each detonator consisted of an electrode and a body into which the explosive charges were pressed. The electrode consisted of 5056 aluminum wire 0.0508 in. diameter and 0.209 in. long, having an 0.0031 in. thick insulation (Formvar) coating on the outer cylindrical surface and having bare flat ends. The body was fabricated of 2024-24 aluminum and was 0.120 in. diameter and 0.249 in. long. An internal cavity of 0.100 in. diameter and 0.170 in. long was provided for the explosive charges. The top of the cavity was tapered inward by a 30 radius shoulder to an 0.0545 in. diameter electrode hole through the top of the body. The electrode was staked into the hole in the body to a depth such that the bottom end just communicated with the upper portion of the tapered top of the charge cavity. The overall detonator length was 0.385 in. The open bottom end of the charge cavity was charged with 10.0+1.0 mg. of the product followed by compression of the charge using a pressure of approximately 12,800 p.s.i. The compressed product was then uniformly covered with 5.0+ 1.0 mg. RD-1333 lead azide and the resulting composite was again compressed at approximately 12,800 p.s.i. The detonators were tested approximately 24 hours later by applying a 20 milliampere current across the electrode and 6 body for 30 seconds. None of the detonators fired. The detonators were tested by applying a voltage through a 2.2 microfarad capacitor across the electrode and body. The detonators fired over a range from 6 to 11 volts in less than 10 microseconds and had firing energies from 39.6 microjoules (396 ergs) to 133.1 microjoules (1331 ergs).

EXAMPLE 3 A suspension of lead azide was prepared as in Example 2 in a 2.5 liter preparation vessel. After about two minutes of stirring, the precipitate was allowed to settle, was decanted and was washed four times with about 1.3 liters of deionized water in each wash by reslurrying and decantation. Following the washes, about 800 ml. deionized water was added to the lead azide and the stirrer restarted. After about two minutes of stirring, 3.89 grams of magnesium powder in sufiicient (S.D.A.3A) denatured alcohol to permit suspension of the powder was poured slowly into the mixture. Immediately thereafter, 54.33 grams of silver nitrate in 200 ml. deionized water solution was added and stirring was continued for one hour. The precipitate was then allowed to settle and was washed four times each time with about 1.3 liters of deionized water by reslurrying and decantation. The solids were then washed twice each time with about 1 liter of denatured alcohol by reslurrying and decantation. The precipitate was then dried under ambient conditions.

Ten detonators were prepared as in Example 2 except that the bottom end of the electrode had a slightly resistive amorphous chromate film of about 0.00002 in. thickness applied to its surface. The film was applied by known methods by immersing the bottom end of each electrode, prior to assembly into the body, for about 3 minutes under ambient conditions in a stirred solution of about 3.0 gm. Alodine 1200s (a source of hexavalent chromium and a source of fluoride) in 200 ml. cold tap water followed by rinsing in cold tap water for about 5 min. and drying at about F. for 4 hours. The treated electrodes were assembled into the detonator bodies and the detonators loaded as in Example 2. The detonators did not fire when subjected to currents of 20 to 48 milliamperes for 30 seconds. All of the detonators fired when voltages of 5 to 7 volts were applied through a 2.2 microfarad capacitor. The firing energies required were from 27.5 microjoules (275 ergs) to 53.9 microjoules (539 ergs).

A third set of ten detonators were prepared as above and having a slightly resistive film on the end of the electrode. The firing voltages were from 4 to 7 volts and the firing energies were from 17.6 microjoules (176 ergs) to 53.9 microjoules (539 ergs).

EXAMPLE 4 Product was prepared as in Example 3 and tested in a set of 10 detonators prepared as in Example 3 and having a slightly resistive film applied to the bottom end of each electrode. The detonators fired over a range of from 3 to 7 volts applied through a 2.2 microfarad capacitor. The firing energies were from 9.9 microjoules (99 ergs) to 53.9 microjoules (539 ergs).

EXAMPLE 5 Product was prepared as in Example 3 in detonators having a slightly resistive film applied to the bottom end of each electrode except that the loading of RD-1333 lead azide was increased to about 28 mg. and a base load of about 18.5 mg. of cyclotetramethylenetetranitramine was compressed at 12,800 p.s.i. pressure onto the RD-1333 lead azide. The electrode-body-assembly was inserted into the open top of an outer protective cup of 202-T4 alumi num, having Walls about 0.022 in. thick and an overall cup height of about 0.270 in. The top of the cup was crimped to the top of the body and the cup-body-electrode area sealed by applying an epoxy resin sealant. The complete detonators were tested as in Example 3 and the firing voltages were from 4 to 8 volts.

What is claimed is:

1. A process for the preparation of an ignition composition comprising an essentially homogeneous particle mixture containing about 90-45% by weight of particles of a metallic azide and about 10-55% by Weight of silver particles comprising reducing dissolved silver nitrate with finely-divided magnesium in the presence of a substantially aqueous suspension of a metallic azide selected from the class consisting of lead azides and silver azides to precipitate essentially all the silver from the nitrate as metal and thereafter recovering the intimate mixture of silver and azide thus obtained.

2. The process of claim 1 wherein the azide is lead azide.

3. The process of claim 2 wherein the finely-divided magnesium is added to the substantially aqueous suspension of lead azide before the silver nitrate is added to said suspension.

4. The process of claim 3 wherein the lead azide and silver nitrate are present in amounts such that said particle mixture contains 25-35% by Weight of silver.

5. The process of claim 3 wherein said particle mixture contains up to 5% by Weight based upon the total weight of said mixture of a graining agent.

6. The process of claim 2 wherein the finely-divided magnesium is added to the substantially aqueous suspension of lead azide after the silver nitrate is added to said suspension.

7. The process of claim 1 wherein the azide is silver azide.

8. The process of claim 3 wherein the lead azide is dextrinated lead azide.

9. The process of claim 6 wherein the lead azide is dextrinated lead azide.

References Cited CARL D. QUARFORTH, Primary Examiner E. A. MILLER, Assistant Examiner US. Cl. X.R. 14994, 108 

